Method for preparing 4B-amino-4&#39;-demethyl-4-desoxypodophyllotoxin

ABSTRACT

The invention relates to a method for synthesizing 4β-amino-4′-demethyl-4-desoxypodophyllotoxin of formula (1), characterized by comprising the following successive steps: a) reacting, in a pure weak acid or in a mixture consisting of acid, water and of organic solvent, without another solvent, at a temperature higher than the ambient temperature, thiourea with 4β-halogenoacetamido-4′-demethyl-4-desoxypodophyllotoxin, and; b) recovering the 4β-amino-4′-demethyl-4-desoxypodophyllotoxin.

CROSS REFERENCE WITH PCT APP

The present application is a 37 C.F.R. §1.53(b) divisional of Ser. No.11/988,942 filed Jan. 17, 2008. Application Ser. No. 11/988,942 claimspriority under 35 U.S.C. §371 of International Application No.PCT/EP2006/064420 filed on Jul. 19, 2006. Priority is also claimed toFrench application 0507642 filed on Jul. 19, 2005. The entire contentsof each of these applications is hereby incorporated by reference.

The present invention relates to a method for preparation of4β-amino-4′-demethyl-4-desoxypodophyllotoxin of formula 1, from4β-halogenoacetamido-4β-demethyl-4-desoxypodophyllotoxine of formula 3(X=Cl, Br or I), by cleavage in the presence of thiourea and an acid. Inparticular, the present invention relates to a method for preparation of4β-amino-4′-demethyl-4-desoxypodophyllotoxin of formula 1, from4′-demethylepipodophyllotoxine of formula 2, via4β-halogenoacetamido-4′-demethyl-4-desoxypodophyllotoxin of formula 3(X=Cl, Br or I).

4β-amino-4′-demethyl-4-desoxypodophyllotoxin is a synthesis intermediaryuseful in preparation of anti-cancer compounds (French patentapplication No. 0404053).

The preparation strategy of this intermediary is based on thetransformation of 4′-demethylepipodophyllotoxin (formula 2) into4β-azido-4′-demethyl-4-desoxypodophyllotoxin of formula 4, then bycatalytic reduction of this azide derivative into an amino derivative offormula 1. The problem of this transformation is the lack ofstereoselectivity of the transformation of the active derivative inposition 4 (benzyl position), providing the mixture of azides α and β offormula 4. This problem has been partly resolved in J. Med. Chem. 1991,34, 3346, by using sodium azide and trifluoroacetic acid. But it provednecessary to purify the azido intermediary of formula 4, bychromatography, and the catalytic reduction product, that is, the aminoof formula 1. Another method was described in Chinese Chemical Letters1993, 4 (4) 289. These authors utilise the azide method, but havingazothydric acid HN3 (prepare in situ) react in the presence of BF₃etherate at −10˜−15° C. The results from these authors indicate goodstereoselectivity of the transformation, with a yield at least equal to80%. A transformation method of the azido of formula 4 into amino offormula 1 is likewise described in Tet. Let. 1999, 40, 1967 and Tet.Let. 2000, 41, 11A2>. These authors utilise Samarium iodide in t-BuOHand THF, or else the FeSO₄.7H₂O/NH₃ couple. Recently, Bioorg. Med. Chem.2003, 11, 5135 confirmed necessary chromatographic purification. Theyobtain the amino of formula 1 with a yield of 70%.

Despite all else these methods pose two problems. 1) the use of adangerous azide derivative, potentially explosive, especially duringlarge-scale usage for industrial preparation of a drug and 2) thenecessary passage through one or even 2 chromatography stages to providean amino compound of formula 1 of good quality to later prepare thefinished product, an anti-cancer drug, which represent cumbersome stageson an industrial scale.

The object of the present invention is to resolve these two problems,not using dangerous or explosive compounds, and without the need forchromatographic purification stages.

4β-halogenoacetamido-4′-demethyl-4-desoxypodophyllotoxin, theintermediary of formula 3, is an already known compound (French patentapplication No. 0404053, WO2004/073375). Similarly, the passage of thecompound of formula 2 to the compound of formula 3 is already known(French patent application No. 0404053). The object of the invention isthus a method for synthesising the compound of formula 1 from thecompound of formula 3.

In classic organic synthesis, cleavage of chloracetamides to gain aminesis executed by treating a chloracetamido derivative of tertiary amide bythiourea in ethanol in the presence of acetic acid in an optimalproportion of 5:1 (A. Jirgensons et al., Synthesis 2000, 1709). In thisreaction, ethanol and acetic acid used contain no water. This method hasnever been applied podophyllotoxine in series and is unsuitable. Infact, applied in the case of the compound of formula 3, the method astaught results after 10 hours under reflux in transformation of lessthan 10% of the primary material involved (compound of formula 3), andthe reactional intermediary (X=S-isothiouronium in the form of chloride)no longer reacts (et comparative example). A longer reaction time isunfavourable in terms of purity, with the appearance of secondaryproducts.

It was necessary to adapt and improve the operating mode to obtain thepreferred transformation. Surprisingly, the inventors determined amethod for synthesis of the compound of formula 1, from the compound offormula 3, resulting in the compound of formula 1 with good purity,without an additional purification stage (chromatography in particular).

The object of the present invention is thus a method for synthesis of4β-amino-4′-demethyl-4-desoxypodophyllotoxin of formula 1

characterised in that it comprises the following successive stages:

a) reaction, in a pure weak acid, with no other solvent, at atemperature higher than ambient temperature, of thiourea with4β-halogenoacetamido-4′-demethyl-4-desoxypodophyllotoxin of formula 3

in which X represents a halogen atom selected from the group made up bychlorine, bromine and iodine, advantageously chlorine;

b) recovery of 4β-amino-4′-demethyl-4-desoxypodophyllotoxin.

Relative to the cleavage method of chloracetamides described in theprior art (A. Jirgensons et al., Synthesis 2000, 1709), it was foundthat the operation can be conducted in a pure weak acid, that is,without water or other organic solvent. In terms of the presentinvention, use of the term “acid” makes reference to the Bronsteddefinition, specifically a chemical type capable of assigning a H⁺proton. A weak acid is an acid which does not fully dissolve in water,as opposed to a strong acid.

The weak acid advantageously has a value of pKa between 4 and 6 at 25°C. In particular, the weak acid is advantageously a carboxylic acid offormula 5 R—COOH, in which R represents hydrogen or an alkyl radical inC1-C2. Heavier acids are no longer usable as solvent, or do not have thesuitable olfactive characteristics (butyric acid in particular). Moreparticularly, the weak acid is selected from the group made up by formicacid, acetic acid or propionic acid, preferably acetic acid. In thefollowing, the given proportions between the different compoundscorrespond to the proportions of quantities involved for thesecompounds, unless indicated otherwise.

Within the scope of the present invention, the expression “pure weakacid” means that this acid is glacial, that is, devoid of water. Theexpression “with no other solvent” means that the reactional medium ofstage a) comprises only pure weak acid, the compound of formula 3 andthiourea, and accordingly does not comprise water or any other solvent,such as alcohol or organic solvent.

During stage a), the reactional medium is advantageously heated to atemperature greater than 60° C., more advantageously between 60 and 100°C. Another characteristic of the invention is that the pure weak acidused serves as solvent to the reaction.

The molar ratio between4β-halogenoacetamido-4′-demethyl-4-desoxypodophyllotoxin and the weakacid is at least 0.5. The molar ratio between4β-halogenoacetamido-4′-demethyl-4-desoxypodophyllotoxin and thiourea isadvantageously between 0.5 and 1. According to an advantageous variantof the invention, during stage a)4β-halogenoacetamido-4′-demethyl-4-desoxypodophyllotoxin is placed incontact with the pure weak acid prior to addition of thiourea. Accordingto an even more advantageous variant,4β-halogenoacetamido-4′-demethyl-4-desoxypodophyllotoxin and the pureweak acid are placed in contact,4β-halogenoacetamido-4′-demethyl-4-desoxypodophyllotoxin beingadvantageously in suspension in the pure weak acid, and the reactionalmedium is heated to the desired temperature prior to addition ofthiourea at this temperature.

The reaction time of stage a) is advantageously between 1 and 3 hours.In the case of the pure acetic acid, the reaction time of stage a) isaround 2 hours.

Following stage a), the final product of formula 1 precipitates in thereactional medium. It is recovered during stage b) by any techniqueknown to the expert, where simple filtration and drying according tostandard methods suffice in particular.

Once filtered and dried according to the usual methods, the compound offormula 1, in the form of hydrochloride, hydrobromide or hydriodide, isobtained with an average molar yield greater than 85%, advantageouslygreater than 90% based on the molar quantity of compound of formula 3used. In the case of pure acetic acid, the compound of formula 1, in theform of hydrochloride, hydrobromide or hydriodide, is obtained with anaverage molar yield of 93% based on the molar quantity of compound offormula 3 used.

The compound of formula 1 is advantageously obtained with a degree ofpurity greater than 90%, more advantageously greater than or equal to95%.

The compound of formula 1, obtained in the form of hydrochloride,hydrobromide or hydriodide, is pure and requires no additional stage ofchromatographic purification. It can be utilised directly for laterstages of synthesis, representing a major advantage from the viewpointof preparation on an economic and industrial scale.

The object of the present invention is likewise a method for synthesisof 4β-amino-4′-demethyl-4-desoxypodophyllotoxin of formula 1

characterised in that it comprises the following successive stages:

i) reaction, in a mixture of acid, water and organic solvent, at atemperature greater than ambient temperature, of thiourea with4β-halogenoacetamido-4′-demethyl-4-desoxydophyllotoxine of formula 3

in which X represents a halogen atom selected from the group made up bychlorine, bromine and iodine, advantageously chlorine;

ii) recovery of 4β-amino-4′-demethyl-4-desoxypodophyllotoxin.

In relation to the cleavage method of the chloracetamides described inthe prior art (A. Jirgensons et al., Synthesis 2000, 1709), it has beenfound that the addition of water to the reactional medium favouredreaction by fully taking up the primary material, without the appearanceof degradation products.

The reactional medium advantageously contains no other solvent orreagent. During stage i), the reactional medium is advantageously heatedto a temperature greater than 60° C., more advantageously between 60 and100° C. The molar ratio between4β-halogenoacetamido-4′-demethyl-4-desoxypodophyllotoxin and thiourea isadvantageously between 0.5 and 1. During stage i),4β-halogenoacetamido-4′-demethyl-4-desoxypodophyllotoxin isadvantageously placed in contact with the mixture of acid, water andorganic solvent prior to addition of thiourea. Even more advantageously,4β-halogenoacetamido-4′-demethyl-4-desoxypodophyllotoxin and the mixtureof acid, water and organic solvent are placed in contact,4β-halogenoacetamido-4′-demethyl-4-desoxypodophyllotoxin beingadvantageously in suspension in said mixture, and the reactional mediumis heated to desired the temperature prior to addition of thiourea atthis temperature.

The organic solvent used in the second method according to the inventionis advantageously a hydrosoluble organic solvent, more advantageouslyselected from the group made up by cyclic ethers, in particular dioxane,alcohols, in particular methanol, ethanol, propanol and isopropanol, andN,N-dimethylacetamide (DMA), dimethylformamide (DMF) andN-methylpyrrolidone (NMP).

So, relative to the cleavage method of the chloracetamides described inthe prior art 1 (A. Jirgensons et al., Synthesis 2000, 1709), it haslikewise been found that the operation can be conducted in the presenceof an organic solvent, such as dioxane or DMA, in place of ethanol, inthe presence of water.

According to a first advantageous variant of the second method accordingto the invention, the organic solvent is an alcohol, advantageouslyethanol.

Within the scope of this first variant, the acid is advantageously astrong acid, in particular selected from the group made up byhydrochloric acid, sulphuric acid and phosphoric acid. Thealcohol/(eau+strong acid) volumetric ratio is advantageously 2 to 5/0.5to 2, more advantageously 2.5/1, the strong acid being once or twicenormal (normality between 1 and 2). The compound of formula 1 isadvantageously obtained with a molar yield greater than 80%, moreadvantageously greater than 85%, advantageously equal to 90%. Thereaction time is advantageously greater than 8 hours, though less than10 hours, even more advantageously around 9 hours.

The compound of formula 1 is likewise advantageously obtained with adegree of purity greater than 90%, advantageously 95%.

Alternatively, within the scope of this first variant, the acid isadvantageously a weak acid, in particular a carboxylic acid of formula 5R—COOH, in which R represents hydrogen or an alkyl radical in C1-C2. Theheavier acids are no longer usable as solvent, or do not have thesuitable olfactive characteristics (butyric acid in particular). Moreparticularly, the weak acid is selected from the group made up by formicacid, acetic acid or propionic acid, preferably acetic acid. Thealcohol/water/weak acid volumetric ratio is advantageously 2 to 10/0.5to 2/0.5 to 2, more particularly 5/1/1. More particularly, theethanol/water/acetic acid volumetric ratio is advantageously 5/1/1. Thecompound of formula 1 is advantageously obtained with a molar yieldgreater than 55%, advantageously equal to 60%. The reaction time isadvantageously greater than 8 hours, though less than 11 hours, evenmore advantageously around 10 hours.

The compound of formula 1 is advantageously obtained with a degree ofpurity greater than 90%, advantageously 95%.

According to a second advantageous variant of the second methodaccording to the invention, the organic solvent is a cyclic ether, inparticular dioxane, and DMA, DMF or NMP.

The cyclic ether (dioxane) or DMA, DMF, NMP/water/weak acid (aceticacid) volumetric ratio is advantageously 2 to 10/0.5 to 2/0.5 to 2, moreparticularly 5/1/1. The dioxane or DMA, DMF, NMP/water/acetic acidvolumetric ratio is advantageously 5/1/1

The compound of formula 1 is advantageously obtained with a molar yieldgreater than 60%, more advantageously greater than 65%, advantageouslyequal to 70%. The reaction time is advantageously greater than 4 hours,though less than 10 hours, even more advantageously around 5-6 hours.

The compound of formula 1 is thus advantageously obtained with a degreeof purity greater than 90%, advantageously 95%.

In the cases studied, the final product precipitates in the reactionalmedium. It is recovered during stage b) by any technique known to theexpert, where simple filtration and drying according to the usualmethods suffice in particular.

The compound of formula 1 obtained in the form of hydrochloride,hydrobromide or hydriodide, is pure and requires no additional stage ofchromatographic purification. It can be utilised directly for latersynthesis stages, representing a major advantage from the viewpoint ofpreparation on an economic and industrial scale.

Within the scope of the first or second method according to theinvention, 4β-halogenoacetamido-4′-demethyl-4-desoxypodophyllotoxin offormula 3 is advantageously obtained by reaction of4′-demethylepipodophyllotoxine of formula 2

with a halogenoacetonitrile of formula 6 X—CH2-C≡N, in which Xrepresents a halogen atom selected from the group made up by chlorine,bromine and iodine, in acid medium. A Ritter reaction ensues to supply4β-chloroacetamido-4′-demethyl-4-desoxypodophyllotoxin directly viacrystallisation on completion of reaction with a yield advantageouslygreater than 80%, even more advantageously greater than 90%.

This intermediary exclusively has stereochemistry β on carbon inposition 4. The problem of stereochemistry is resolved at this stage.The purity of this intermediary is such that it can be utilised withoutsubsequent purification in the cleavage stage to provide4β-amino-4′-demethyl-4-desoxypodophyllotoxin of formula 1.

The 4′-demethylepipodophyllotoxine of formula 2 (prepared according tothe method described in the patent FR 2 742 439) is advantageouslytreated by chloroacetonitrile, a common an inexpensive reagent, withsulphuric acid. 4β-chloroacetamido-4′-demethyl-4-desoxydophyllotoxine isthen advantageously obtained with a yield of 93%.

Once it is filtered and dried according to standard methods, thecompound of formula 1, in the form of hydrochloride, hydrobromide orhydriodide, is obtained, within the scope of the utilisation of pureglacial acetic acid (neither water nor any other organic solvent, thefirst method according to the invention) with an average molar yield of86% based on the molar quantity of 4′-demethylepipodophyllotoxine(formula 2) used, that is, over 2 stages (from the compound of formula 2to the compound of formula 3, then from the compound of formula 3 to thecompound of formula 1).

The following examples show the operating techniques utilised.

EXAMPLE 1 Preparation of4β-chloroacetamido-4′-demethyl-4-desoxypodophyllotoxin (formula 3)

Dropwise, 0.5 mL of concentrated sulphuric acid is added at ambienttemperature to the suspension of 30 g (0.075 mole) of4′-demethylepipodophyllotoxin in 47.5 mL (0.75 mole) ofchloroacetonitrile. This is stirred at this temperature for 1 hour, whendissolution then reprecipitation are observed. 300 mL of 2-propanol areadded. The precipitate is filtered and rinsed with 200 mL of propanol-2and water to return to pH 7. The white solid obtained is dried undervacuum at 40° C. to give 32.9 g of the compound chloroacetamido offormula 3, or a molar yield of 93%.

Melting point F=240° C.

RMN analysis of the proton: ¹H RMN (DMSO) [delta] 8.65 (d, IH, J=7 Hz,NH), 8.26 (s, IH, 4′-OH), 6.78 (S, IH, H5), 6.54 (s, IH, H8), 6.24 (s,2H, H2, H6), 5.99 (d, 2H, J=11.3 Hz, OCH₂O), 5.17 (dd, IH, J=4.56 and 7Hz, H4), 4.5 l (d, IH, J=5.2 Hz, Hi), 4.29 (1, IH, J=8 Hz, Hlla), 4.10(s, 2H, CH₂Cl), 3.97 (m, IH, H3), 3.78 (dd, IH, J=8 Hz and 10 Hz, Hnb),3.63 (s, 6H, 2×OCH₃), 3.15 (dd, IH, J=5.2 and 14 Hz, H2).

The other halogenoacetamides (X=Br, I) are obtained similarly by usingbromoacetonitrile or iodoacetonitrile.

COMPARATIVE EXAMPLE Preparation of4-amino-4′-demethyl-4-desoxypodophyllotoxin (formula 1)

Method with ethanol:acetic acid 5:1 (according to Synthesis 2000, 1709)

Table 1: Entry 1.

A suspension of 0.5 g (1.05 mmoles) of4β-chloroacetamido-4′-demethyl-4-desoxypodophyllotoxin obtained inexample 1, in a mixture of 2.5 mL of ethanol and 0.5 mL of glacialacetic acid is brought to 80° C. with agitation. 0.12 g (1.57 mmoles) ofthiourea is added at once. This is agitated at this temperature for 10hours. Analysis of the reactional medium, estimated by thin-layerchromatography, reveals only less than 10% of the desired product4β-amino-4′-demethylepipodophyllotoxin (formula 1), of the presence ofthe intermediary isothiouronium (X=S-isothiouronium) which no longerreacts, and degradation products.

EXAMPLE 2 Preparation of 4β-amino-4′-demethyl-4-desoxypodophyllotoxin(formula 1)

Method with Pure Glacial Acetic Acid—First Method According to theInvention.

Table 1: Entry 2.

A suspension of 17 g (0.0358 mole) of4β-chloroacetamido-4′-demethyl-4-desoxypodophyllotoxin obtained inexample 1, in 75 mL of glacial acetic acid is brought to 80° C. withagitation. 4.2 g (0.0537 mole) of thiourea is added at once. This isagitated at this temperature for 1 h 30, when dissolution thenreprecipitation are observed. The reactional medium is hot-filtered,rinsed with 75 mL of glacial acetic acid and diisopropylic ether. Thewhite solid obtained is dried under vacuum at 40° C. to give 14.6 g ofthe compound of formula 1 in its hydrochloride form corresponding to amolar yield of 93%.

Melting point: F>260° C.

RMN analysis of the proton: ¹H RMN (DMSO) [delta] 8.63 (m, 2H), 8.32 (m,IH), 7.23 (s, IH, H5), 6.60 (s, IH, H8), 6.8 (s, 2H, H2, H6), 6.05 (d,2H, J=2.1 Hz, OCH₂O), 4.73 (d, IH, J=4.5 Hz, H4), 4.56 (d, IH, J=5.2 Hz,Hi), 4.34 (m, 2H, Hna and Hiib), 3.65 (dd, IH, J=5.2 Hz, H₂), 3.62 (s,6H, 2×OCH₃), 3.06 (m, IH, H3).

EXAMPLE 3 Preparation of 4β-amino-4′-demethyl-4-desoxypodophyllotoxin(formula 1)

Method with Ethanol and Hydrochloric Acid IN—Second Method According tothe Invention, First Variant, First Alternative.

Table 1: Entry 3.

A suspension of 0.5 g (1.05 mmoles) of4β-chloroacetamido-4′-demethyl-4-desoxypodophyllotoxin obtained inexample 1, in a mixture of 2.5 mL of ethanol and 1 mL of hydrochloricacid IN is brought to 80° C. with agitation. 0.12 g (1.57 mmoles) ofthiourea is added at once. This is agitated at this temperature for 9hours when dissolution then reprecipitation are observed. The cooledreactional medium is filtered, rinsed with ethanol and diisopropylicether. The white solid obtained is dried under vacuum at 40° C. to give0.4 g of the compound of formula 1 in its hydrochloride formcorresponding to a molar yield of 90%.

Melting point: F>260° C.

EXAMPLE 4 Preparation of 4β-amino-4′-demethyl-4-desoxypodophyllotoxin(formula 1)

Method with ethanol:water:acetic acid (5:1:1)

Second Method According to the Invention, First Variant, SecondAlternative.

Table 1: Entry 4.

A suspension of 0.5 g (1.05 mmoles) of4β-chloroacetamido-4′-demethyl-4-desoxypodophyllotoxin obtained inexample 1, in a mixture of 2.5 mL of ethanol, 0.5 mL of water and 0.5 mLof glacial acetic acid is brought to 80° C. with agitation. 0.12 g (1.57mmoles) of thiourea is added at once. This is agitated at thistemperature for 10 hours when dissolution then reprecipitation areobserved. The cooled reactional medium is filtered, rinsed with ethanoland diisopropylic ether. The white solid obtained is dried under vacuumat 40° C. to give 0.27 g of the compound of formula 1 in itshydrochloride form corresponding to a molar yield of 600. Melting point:F>260° C.

EXAMPLE 5 Preparation of 4β-amino-4′-demethyl-4-desoxypodophyllotoxin(formula 1)—Method with Solvent (DMA, Dioxane)/Water/Acetic Acid—SecondMethod According to the Invention, Second Variant Table 1: Entry 5.

A suspension of 0.5 g (1.05 mmoles) of4β-chloroacetamido-4′-demethyl-4-desoxypodophyllotoxin obtained inexample 1, in a mixture of 2.5 mL of dioxane or DMA, 0.5 mL of water and0.5 mL of glacial acetic acid is brought to 80° C. with agitation. 0.12g (1.57 mmoles) of thiourea is added at once. This is agitated at thistemperature for 5 to 6 hours when dissolution then reprecipitation areobserved. The cooled reactional medium is filtered, rinsed with2-propanol and diisopropylic ether. The white solid obtained is driedunder vacuum at 40° C. to give 0.31 g of the compound of formula 1 inits hydrochloride form corresponding to a molar yield of 70%.

Melting point: F>260° C.

The results of the assays of the comparative example and examples 2 to 5are summarised in the following Table 1:

Reaction Yield of Entry Conditions time compound 1 Purity 1 Ethanol/ 10h <10% Presence of acetic acid non- first (5/1) isolated material, CCMreactional evaluation intermediary, degradation products 2 Pure glacial 2 h  93% >95% acetic acid at 80° C. 3 Ethanol/  9 h  90%  95%hydrochloric acid 1N 4 Ethanol/ 10 h  60%  95% water/ acetic acid(5/1/1) 5 Solvent: 5-6 h   70%  95% dioxane or DMA/acetic acid/water(5/1/1)

Table 1 shows the major advantage of using pure glacial acetic acid at80° C. with a short reaction time of 2 h to provide the desired productwith an excellent yield in a highly satisfactory state of purity forsubsequent use in the synthesis of anti-cancer compounds.

1. A method for synthesising4β-amino-4′-demethyl-4-desoxypodophyllotoxin of formula 1

comprising the following successive stages: i) reaction, in a mixture ofacid, water and organic solvent, at a temperature greater than ambienttemperature, of thiourea with4β-halogenoacetamido-4′-demethyl-4-desoxypodophyllotoxin of formula 3

in which X represents a halogen atom selected form the group composed ofchlorine, bromine and iodine, advantageously chlorine; ii) recovery of4β-amino-4′-demethyl-4-desoxypodophyllotoxin.
 2. The method as claimedin claim 1, wherein during stage i), the reaction medium is heated to atemperature between 60 and 100° C.
 3. The method as claimed in claim 1,wherein, during stage i),4β-halogenoacetamido-4′-demethyl-4-desoxypodophyllotoxin is placed incontact with the mixture of acid, water and organic solvent prior toaddition of thiourea.
 4. The method as claimed in claim 1, wherein theorganic solvent is a hydrosoluble organic solvent.
 5. The method asclaimed in claim 4, wherein the organic solvent is selected form thegroup composed of cyclic ethers, alcohols, N,N-dimethylacetamide,dimethylformamide, N-methylpyrolidone.
 6. The method as claimed in claim5, wherein the cyclic ether is dioxane.
 7. The method as claimed inclaim 5, wherein the alcohol is ethanol.
 8. The method as claimed inclaim 7, wherein the acid is a strong acid.
 9. The method as claimed inclaim 8, wherein the strong acid is selected from the group composed ofhydrochloric acid, sulphuric acid and phosphoric acid.
 10. The method asclaimed in claim 1, wherein the acid is a weak acid.
 11. The method asclaimed in claim 10, wherein the weak acid is a carboxylic acid offormula 5 R—COOH, in which R represents hydrogen, an alkyl radical inC₁-C₂.
 12. The method as claimed in claim 11, wherein the weak acid isacetic acid.
 13. The method as claimed in claim 5, wherein the acid isacetic acid.
 14. The method as claimed in claim 13, wherein the ethanolor dioxane or N,N-dimethylacetamide, dimethylformide,N-methylpyrrolidone/water/acetic acid volumetric ratio is 5/1/1.
 15. Themethod as claimed in claim 1, wherein the molar ratio between4β-halogenoacetamido-4′-demethyl-4-desoxypodophyllotoxin and thiourea isbetween 0.5 and
 1. 16. The method as claimed in claim 1, wherein the4β-halogenoacetamido-4′-demethyl-4-desoxypodophyllotoxin of formula 3 isobtained by reaction of 4′-demethylpipodophyllotoxin of formula 2

with a halogenoacetonitrile of formula 6 X—CH₂—C≡N, in which Xrepresents a halogen atom selected from the group composed of chlorine,bromine and iodine, in an acid medium.